The use of electrodes for sensing electrical activity at the surface of living tissue, such as during the performance of an electroencephalograph (EEG), an electromyograph (EMG), an electrocardiograph (EKG) or a galvanic skin response (GSR) procedure is well known. These electrodes and others are also used for stimulating living tissue, e.g., TENS (Transcutaneous Electric Nerve Stimulation), defibrillation, pacing (internal and external), or for transferring energy from electrical devices to the body as in electrocautery. These and other prior electrodes provide resistive coupling to the test subject, so as to facilitate the monitoring of electrical activity therein or contain a metallic conductor in chemical contact with an electrolytic medium.
Resistively coupled electrodes have proved to be generally suitable for their intended purposes, however, these electrodes do possess inherent deficiencies, which detract from their utility. For example, resistively coupled electrodes can consume a lot of power, which is undesirable for battery driven devices. Further, they can generate a substantial amount of heat, which can cause burns in defibrillation applications.
Additionally, there are limitations that may occur with both the sensing and stimulation applications using resistively coupled electrodes. Motion artifact, half-cell potential, and non-linearity or distortions of the signal at the electrode-electrolyte interface are some of the limitations that may occur with sensing applications. In stimulation applications, limitations also include non-uniform current density, spikes in amplitude at the onset of the signal, and resistive power loss. All of which are related to the electrode-patient interface.
The transmission of an electrical signal between an electrode and an ionic medium involves certain capacitive and chemical issues. Current exists in metal as a flow of electrons through the crystal lattice of the material. In contrast, current in an ionic solution requires the movement of cations and/or anions through the solution. The electrical interaction between metal and an ionic solution can occur as a capacitive process, an inductive process, or as a chemical reaction.
Typically, both capacitive and chemical interactions take place during electrical activity between a patient and an electrode. The volume of ionic solution on a metal is called the Helmholtz double layer and contains both the capacitive and chemical reactions. Generally all electrodes have a capacitive component except for silver/silver chloride electrodes, commonly used for ECG sensing, at small currents. Additionally, platinum or other inert metals can transmit signals in a purely capacitive mode, but also at small currents only.
The nature of the reaction for most electrodes depends on multiple factors. Generally, the metal composition of the electrode determines the threshold at which chemical reaction will occur, and what they will be, presuming a saline ionic solution. Most metals, including stainless steel, will produce hydrogen and chlorine gases as a byproduct of the chemical reaction of the metal with the ionic solution. This is undesirable because chlorine gas can possibly irritate the patient's tissue at the anode. Further, these gases can cause corrosion of the electrode itself.
Generally, all electrodes, except for silver/silver chloride electrodes and a few others, have a strong capacitive component. Silver/silver chloride avoids this capacitive component at small currents by “anodal chloridization of the electrode surface”. However, the silver/silver chloride electrodes create a capacitive interference with large currents. Electrochemical polarization of physiological electrodes is an undesirable but seemingly unavoidable phenomenon that detracts from the performance of implanted electronic prosthetic devices. In the case of noble metals, polarization causes a significant waste of stimulation energy at the electrode surface. With non-noble metals, the energy waste is even greater and may involve electrolytic corrosion reactions. Such corrosion may destroy the electrode and may possibly leave toxic residues in body tissues. The electrode-electrolyte interface presents to a cardiac pacemaker a highly capacitive load having multiple time constants of the same order of magnitude as the 1- or 2-millisecond (msec) duration of a pacemaking impulse. Thus, an applied square wave of current on the electrodes does not obey Ohm's law and does not elicit a square wave of voltage, nor is the voltage waveform a constant slope (ramp), as would be expected from a single lumped capacitor. Rather, the voltage rises in less than a microsecond to an initial value and then more slowly, in at least two different time constants, until the end of the pulse. This capacitive interference, complicates stimulation with this type of electrode.
It has been found that platinum electrodes can avoid toxicity since they produce only a small amount of chlorine. However, approximately 60% of the current through a platinum pacemaker electrode occurs through capacitance. Thus existing stimulation electrodes mostly include capacitive effects, however, the capacitance is complex and extremely variable. This capacitance is undesirable for several reasons. The capacitance varies in a nonlinear fashion with a myriad of parameters including temperature and rate of change of the electrical signal coming from the patient. This capacitance degrades the electrical signal coming from the patient and is impossible to model for filtering purposes. Further, the capacitance's resistive component also degrades the electrical signal. There are at least two ways the chemical reactions occurring at the electrode surface affect electrical signals. First, is the formation of gas bubbles, which act as a physical barrier to current passage. Second, the half-cell potential changes with small perturbations in the physical environment, creating electrical noise.
Purely capacitive electrodes solve this problem since they avoid chemical reactions all together, but existing technology limits their applications. An example of a purely capacitive electrode is dispersive electrodes used in electrocautery. These electrodes consist of a sheet of metal and a non-conductive adhesive gel in contact with the skin. The adhesive gel has low conductivity but a high dielectric constant. The metal foil forms one plate of the capacitor and the skin forms the other. The capacitance of these electrodes typically ranges in the Pico farad range. Because the electrocautery unit operates in the 400-kilohertz range, the reactance is low.
Dispersive electrodes also require a low impedance interface. Resistive dispersive electrodes can monitor the adequacy of the contact between the electrode and the patient's body by contact quality monitoring (“CQM”) circuitry in an electrosurgical generator. Current generator systems have safety circuits, which can detect when a resistive electrode does not have good attachment to the body. If something has caused the electrode to be applied without adequate initial contact with the body or some event during surgery has caused the adequate initial contact to become inadequate, these safety circuits will detect that problem and terminate the current being applied.
While existing capacitive electrodes do not have the edge effect (electrical fields on the edge of the electrode) of concern for resistive type dispersive electrodes and the current transfer is much more uniform across the surface of the electrode compared to resistive types, they are not compatible with the above described CQM circuits, and thus when used do not have this protection against inadvertent misapplication of electrocautery units used during electrosurgery. Lossy dielectric designs, such as the design described in U.S. Pat. No. 5,836,942, overcome this problem, but the design's resistive component adds to unwanted heat generation. Problems faced by designers of medical electrodes include minimizing overall heat generation and maximizing uniformity of the current density.
Another disadvantage associated with traditional stimulating electrodes, is they often cause an initial uncomfortable shock before attaining a stable sensation.
In view of the foregoing, it is desirable to provide an electrode suitable for use in EEG, EMG, EKG, and GSR procedures and the like overcoming the disadvantages of the prior art by manipulating the electrode-electrolyte interface of a medical electrode in contact with a biological system and providing a large capacitance in a standard sized electrode. It is desirable to have a substantially capacitive electrode to avoid chemical reactions. Additionally it is desirable to have an electrode with a constant predictable capacitance and that can avoid a half-cell potential.